Smear reduction for a television pickup

Abstract

A television pickup arrangement comprises a solid state pickup device which is read by field transfer. The pickup produces a picture signal which on display shows information smear. It is known to perform a correction by obtaining signal smear information from under an opaque, masking strip on the picture pickup portion of the pickup device. The smear picture signal is stored once in every field period in a memory device which is repeatedly readable at line frequency. The smear picture signal is subtracted from the picture signal to be corrected. If there is movement in the scene in the horizontal direction, an unacceptable smear is, however, still observable. According to the invention, a correction may be effected, using a second memory device. The second memory device may be used for performing a signal averaging operation over several field periods or for storing a smear picture signal so that different smear corrections can be effected in the upper and lower picture halves. In both cases the smear is reduced to an acceptable level.

What is claimed is: 
     
       1. A correction arrangement for reducing smear in a raw television picture signal to produce as an output a corrected television picture signal, said arrangement comprising: delay means having an input and an output, the input for receiving the corrected television picture signal, the output supplying an output signal which is equal to the corrected television picture signal after a delay of one field period;   comparator means for measuring the difference between the raw television picture signal and the delayed corrected television picture signal, said comparator means producing a first output signal if the measured difference exceeds a preselected threshold value and producing a second output signal if the measured difference is less than the preselected threshold value; and   signal averaging means for producing a corrected output signal in response to the output of the comparator means, said corrected output signal being equal to the raw television picture signal if the output of the comparator means is the first output signal, said corrected output signal being equal to a weighted average of the raw television picture signal and the delayed corrected television picture signal if the output of the comparator means is the second output signal, said corrected output signal being the corrected television picture signal.   
     
     
       2. A correction arrangement as claimed in claim 1, characterized in that the signal averaging means comprises: means for attenuating the raw television picture signal by a first preselected factor to produce an attenuated raw television picture signal;   means for attenuating the delayed corrected television picture signal by a second preselected factor to produce an attenuated delayed corrected television picture signal;   means for adding the attenuated raw television picture signal to the attenuated delayed corrected television picture signal to produce a weighted average signal; and   switch means responsive to the output of the comparator means, said switch means having an output for the corrected output signal, said switch means passing the raw television picture signal to the output if the output of the comparator means exceeds the preselected threshold value, said switch means passing the weighted average signal to the output if the output of the comparator means is less than the preselected threshold value.   
     
     
       3. A correction arrangement as claimed in claim 2, characterized in that the first preselected attenuation factor is equal to one minus the second preselected attenuation factor. 
     
     
       4. A correction arrangement for reducing smear in a raw television picture signal to produce as an output a corrected television picture signal, said arrangement comprising: first means for storing a first smear picture signal from a first television field;   second means for storing a second smear picture signal from a second television field which immediately follows the first television field;   means for subtracting the first smear picture signal from the first half of the raw television picture signal of the second television field to produce a first difference signal, said first difference signal being the first half of the corrected television picture signal; and   means for subtracting the second smear picture signal from the second half of the raw television picture signal of the second television field to produce a second difference signal, said second difference signal being the second half of the corrected television picture signal.   
     
     
       5. A correction arrangement as claimed in claim 4, characterized in that the means for subtracting the first and second smear picture signals comprise: switch means having first and second inputs and one output, for switchably connecting one input at a time to the output, the first input for receiving a signal from the means for storing the first smear picture signal, the second input for receiving a signal from the means for storing the second smear picture signal;   delay means having an input and an output, the input for receiving the raw television picture signal, the output supplying an output signal which is equal to the raw television picture signal after a delay of one field period; and   a single subtraction circuit means having first and second inputs and an output, the first input receiving the delayed raw television picture signal, the second input receiving the output of the switch means, the subtraction circuit output providing a signal representing the output of the switch means subtracted from the raw television picture signal.   
     
     
       6. A correction arrangement as claimed in claim 4, characterized in that the arrangement further comprises a solid state pickup device comprising: means for generating first and second smear picture signals for each television field;   means for reading out the first smear picture signal into the first storing means after reading out the associated television field; and   means for reading out the second smear picture signal into the second storing means before reading out the associated television field.   
     
     
       7. A correction arrangement as claimed in claim 4, characterized in that the arrangement further comprises a solid state pickup device comprising: means for generating a smear picture signal for each television field; and   means for nondestructively reading out the smear picture signal during readout of the second half of the associated television field, said nondestructive readout means forming the second means for storing the second smear picture signal.